The hydroxylation of aromatic compounds, such as phenols, in the presence of an oxidizing agent, especially aqueous hydrogen peroxide solution, and of a catalyst leads to the formation of hydroxylated aromatic compounds, such as hydroquinone (HQ) and pyrocatechol (PC) for phenol, but also to the formation of side products, especially tars. To limit the formation of these tars, the conversion of the starting aromatic compound should be limited, and is, for example, in the case of phenol, from 5% to 30%.
More specifically, the demand for hydroquinone is relatively high, especially in the field of polymerization inhibitors. One way of satisfying this increasing demand for hydroquinone would be to orient the hydroxylation reaction of phenol toward the formation of hydroquinone.
The prior art, especially FR 2 071 464 discloses the use of a homogeneous catalyst of optionally modified strong acid type, for the hydroxylation of phenol. The use of this hydroxylation reaction of phenol in the presence of a homogeneous strong acid catalyst leads to the formation of hydroquinone and pyrocatechol in a PC/HQ mole ratio, depending on the acid used, of greater than or equal to 1.5. In general, the use of a homogeneous catalyst leads to the formation of hydroquinone and pyrocatechol in a PC/HQ mole ratio of greater than or equal to 1.
It is also known from the prior art, especially from FR 2 523 575, to use a zeolite of titano-silicalite type of structure MFI or MEL, especially TS-1 or TS-2, respectively, a heterogeneous catalyst, for the hydroxylation of phenol. It is thus possible to obtain hydroquinone and pyrocatechol with a PC/HQ mole ratio of less than 1.3, especially between 0.4 and 1.3, depending on the catalyst or the solvent used in the phenol hydroxylation reaction.
It is, however, worthwhile providing a process for obtaining the lowest possible PC/HQ mole ratio in order to promote the formation of hydroquinone while at the same time maintaining the highest possible yield in an environmentally friendly solvent such as water.
TS-1 and TS-2 are zeolites characterized by the presence of titanium atoms in replacement for silicon atoms in the structure. These zeolites have, respectively, a structure MFI or MEL and are generally obtained by mixing a source of silicon, a source of titanium, a structuring agent and a mineralizing agent, the mixture obtained then being crystallized for 1 to 10 days at a temperature typically close to 175° C. and finally calcined for 3 to 12 hours, at a temperature typically close to 550° C. Processes for preparing TS-1 are especially described in U.S. Pat. No. 4,410,501 or EP 0 311 983.
TS-1 and TS-2 have advantageous catalytic properties and are thus used in many reactions such as the hydroxylation of phenol, the ammoximation of cyclohexanone or the epoxidation of alkenes.
Many studies have been conducted aimed at improving the catalytic performance of titano-silicalite zeolites. These studies have related especially to the crystallization time, the source of silicon, the source of structuring agent, the TPAOH/Si mole ratio, the H2O/Si mole ratio or the Ti/(Ti+Si) mole ratio (Van Der Pol et al., Appl. Catal. A General, 1992 92 93-111).
An optimized process should be found for preparing titanium silicalites such as TS-1 or TS-2, making it possible to improve their catalytic properties, especially in the context of the hydroxylation reaction of aromatic compounds, and in particular of phenol.
One object of the present invention is to provide an improved process for the hydroxylation of aromatic compounds, and in particular of phenol, anisole and para-t-butylphenol.
Another object of the present invention is to provide a process for the hydroxylation of phenol which allows the preparation of hydroquinone and pyrocatechol with a PC/HQ mole ratio of less than 1.4, preferably less than 1.2, more preferably less than 1, preferably strictly less than 0.8 and preferably strictly less than 0.7.
Another object of the present invention is to provide a titano-silicalite zeolite that is suitable for use in this process for the hydroxylation of aromatic compounds.
Other objects will become apparent on reading the description of the invention that follows.